Dear Tom,Why does the sky turn green...

Tom Skilling, chief meteorologist at WGN-TVCHICAGO TRIBUNE

Dear Tom,

Why does the sky turn green when there's going to be a tornado?

Mrs. Frost's 8th grade science class

The greenish color sometimes observed in the clouds of a thunderstorm is the result of hail present within the storm. Chicago National Weather Service meteorologist Paul Merzlock explains that hail filters out the blue colors in sunlight, leaving the light with wavelengths more characteristic of green colors. It's that light that lends a greenish tint to the clouds.

More important than the color of thunderstorm clouds is the relationship between hail and tornadoes. Tornadoes form within the rising-air portion of thunderstorms, and the updrafts of tornadic thunderstorms are usually among the strongest. However, it also takes a powerful updraft to support hail long enough for it to grow large. Thus, the occurrence of large hail (golf-ball size or larger) is a good indicator of the possibility of a tornadic thunderstorm. Note, however, that the presence of large hail and green thunderstorm clouds does not mean a tornado will always occur.

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Tom Skilling is chief meteorologist at WGN-TV. His weather forecasts can be seen Monday through Friday on WGN News at noon and 9 p.m.